A few improvements to make jni_generator work outside of chromium.

* Print a usage message if called with no (valid) arguments.
* Determine the script name by stripping before either 'base' or
    'third_party', or strip nothing if neither name can be found in
    the path.
* Allow overriding the script name, and use this in the tests to be
  path-agnostic.

TBR=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/23702057

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226503 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226503